Element-based descriptions of documents can be found in descriptions prepared in accordance with particular markup languages. For example, the Standard Generalized Markup Language (SGML) was developed and adopted by the International Standards Organization (ISO) in 1986. Another example, the eXtensible Markup Language (XML), also defines rules for describing documents using elements. XML is used currently to define many documents to which access is provided over the Internet.
When a document is described in terms of elements, whether in accordance with SGML, XML, or using a nonstandard approach, the elements are often related to one another in a manner beyond sequence. For example, while a document might consist of only a list of paragraph elements without any other structure, many documents will also include chapters, into which paragraphs are grouped. As an example, a chapter element may include a title element and a number of paragraph elements. The title and paragraph elements are each nested within the chapter element because they begin and end after the chapter element begins, but before it ends.
Databases are used to store and retrieve information. One type of information that can be stored is element-based document descriptions. For example, a database user may desire to store all the XML documents that are located on a website. The independent elements of the XML document, by themselves, do not contain the same information as the XML document because the sequential and nesting relationships between the elements are information included in the XML document. It is useful to store the relationship information in a database in addition to the information concerning the individual elements.